Add validation and consolidation of fix-it hints

The first aspect of this patch is to add some checking of fix-it hints.
The idea is to put this checking within the rich_location machinery,
rather than requiring every diagnostic to implement it for itself.

The fixits within a rich_location are "atomic": all must be valid for
any to be applicable.

We reject any fixits involving locations above
LINE_MAP_MAX_LOCATION_WITH_COLS.

There's no guarantee that it's sane to modify a macro, so we reject
any fix-its that touch them.

For example, note the attempt to provide a fix-it for the definition
of the macro FIELD:

spellcheck-fields-2.c: In function ‘test_macro’:
spellcheck-fields-2.c:26:15: error: ‘union u’ has no member named ‘colour’; did you mean ‘color’?
 #define FIELD colour
               ^
               color
spellcheck-fields-2.c:27:15: note: in expansion of macro ‘FIELD’
   return ptr->FIELD;
               ^~~~~

After this patch, the fixit is not displayed:

spellcheck-fields-2.c: In function ‘test_macro’:
spellcheck-fields-2.c:26:15: error: ‘union u’ has no member named ‘colour’; did you mean ‘color’?
 #define FIELD colour
               ^
spellcheck-fields-2.c:27:15: note: in expansion of macro ‘FIELD’
   return ptr->FIELD;
               ^~~~~

We might want some way for a diagnostic to opt-in to fix-its that
affect macros, but for now it's simplest to reject them.

The other aspect of this patch is fix-it consolidation: in some cases
neighboring fix-its can be merged.  For example, in a diagnostic to
modernize old-style struct initializers from:

 struct s example = {
- foo: 1,
+ .foo = 1,
 };

one approach would be to replace the "foo" with ".foo" and the ":"
with " =".  This would give two "replace" fix-its:

  foo: 1,
  --- FIXIT 1
  .foo
     - FIXIT 2
     =

This patch allows them to be consolidated into a single "replace" fix-it:

  foo: 1,
  ----
  .foo =

gcc/ChangeLog:
	* diagnostic-show-locus.c
	(selftest::test_fixit_consolidation): New function.
	(selftest::diagnostic_show_locus_c_tests): Call it.
	* gcc-rich-location.h (gcc_rich_location): Eliminate unused
	constructor based on source_range.

gcc/testsuite/ChangeLog:
	* gcc.dg/spellcheck-fields-2.c (test): Move
	dg-begin/end-multiline-output within function body.
	(test_macro): New function.

libcpp/ChangeLog:
	* include/line-map.h (rich_location): Eliminate unimplemented
	constructor based on source_range.
	(rich_location::get_last_fixit_hint): New method.
	(rich_location::reject_impossible_fixit): New method.
	(rich_location): Add fields m_line_table and
	m_seen_impossible_fixit.
	(fixit_hint::maybe_append_replace): New pure virtual function.
	(fixit_insert::maybe_append_replace): New function.
	(fixit_replace::maybe_append_replace): New function.
	* line-map.c (rich_location::rich_location): Initialize
	m_line_table and m_seen_impossible_fixit.
	(rich_location::add_fixit_insert): Call
	reject_impossible_fixit and bail out if true.
	(column_before_p): New function.
	(rich_location::add_fixit_replace): Call reject_impossible_fixit
	and bail out if true.  Attempt to consolidate with neighboring
	fixits.
	(rich_location::get_last_fixit_hint): New method.
	(rich_location::reject_impossible_fixit): New method.
	(fixit_insert::maybe_append_replace): New method.
	(fixit_replace::maybe_append_replace): New method.

From-SVN: r239789

libcpp/include/line-map.h

@@ -1367,9 +1367,6 @@ class rich_location
   /* Constructing from a location.  */
   rich_location (line_maps *set, source_location loc);
 
-  /* Constructing from a source_range.  */
-  rich_location (source_range src_range);
-
   /* Destructor.  */
   ~rich_location ();
 
@@ -1411,12 +1408,17 @@ class rich_location
 
   unsigned int get_num_fixit_hints () const { return m_num_fixit_hints; }
   fixit_hint *get_fixit_hint (int idx) const { return m_fixit_hints[idx]; }
+  fixit_hint *get_last_fixit_hint () const;
+
+private:
+  bool reject_impossible_fixit (source_location where);
 
 public:
   static const int MAX_RANGES = 3;
   static const int MAX_FIXIT_HINTS = 2;
 
 protected:
+  line_maps *m_line_table;
   unsigned int m_num_ranges;
   location_range m_ranges[MAX_RANGES];
 
@@ -1427,6 +1429,7 @@ protected:
 
   unsigned int m_num_fixit_hints;
   fixit_hint *m_fixit_hints[MAX_FIXIT_HINTS];
+  bool m_seen_impossible_fixit;
 };
 
 class fixit_hint
@@ -1440,6 +1443,10 @@ public:
   virtual bool affects_line_p (const char *file, int line) = 0;
   virtual source_location get_start_loc () const = 0;
   virtual bool maybe_get_end_loc (source_location *out) const = 0;
+  /* Vfunc for consolidating successor fixits.  */
+  virtual bool maybe_append_replace (line_maps *set,
+				     source_range src_range,
+				     const char *new_content) = 0;
 };
 
 class fixit_insert : public fixit_hint
@@ -1452,6 +1459,9 @@ class fixit_insert : public fixit_hint
   bool affects_line_p (const char *file, int line);
   source_location get_start_loc () const { return m_where; }
   bool maybe_get_end_loc (source_location *) const { return false; }
+  bool maybe_append_replace (line_maps *set,
+			     source_range src_range,
+			     const char *new_content);
 
   source_location get_location () const { return m_where; }
   const char *get_string () const { return m_bytes; }
@@ -1478,6 +1488,9 @@ class fixit_replace : public fixit_hint
     *out = m_src_range.m_finish;
     return true;
   }
+  bool maybe_append_replace (line_maps *set,
+			     source_range src_range,
+			     const char *new_content);
 
   source_range get_range () const { return m_src_range; }
   const char *get_string () const { return m_bytes; }